1. Field of the Invention
This invention relates to thermosetting resin compositions useful as underfill sealants for mounting to a circuit board semiconductor chips or semiconductor device packages, which have a semiconductor chip on a carrier substrate. Reaction products of these compositions demonstrate improved adhesion after exposure to elevated temperature conditions, improved resistance to moisture absorption and improved resistance to stress cracking.
2. Brief Description of Related Technology
In recent years, the popularity of smaller-sized electronic appliances has made desirable size reduction of semiconductor devices. As a result, chip packages are becoming reduced in size to substantially that of the bare die themselves. Such smaller-sized chip packages improve the characteristics of the microelectronic device in which it is used, while retaining many beneficial operating features. This serves to protect semiconductor bare chips, and increases their reliability and useful life.
Ordinarily, chip assemblies are connected to electrical conductors on a circuit board by use of solder connection or the like. However, when the resulting chip/circuit board structure is subjected to conditions of thermal cycling, reliability becomes suspect due to fatigue of the solder connection between the circuit board and the chip assembly. Recent manufacturing advances provide a sealing resin (often referred to as underfill sealant) in the space created by the mounting of a semiconductor device, such as a chip scale package (“CSP”)/ball grid array (“BGA”)/land grid array (“LGA”) assembly or a flip chip (“FC”) assembly, onto a circuit board to relieve stresses caused by thermal cycling. Underfill sealants have been seen to improve heat shock properties and enhance the reliability of such structures.
Of course, curable resin compositions generally are known. However, a perception to many end users of such resin compositions in microelectronics applications, such as underfill sealants, is their inability to retain adhesion after exposure to temperatures often reached during the solder reflow cycle. That is, due to the difference of the coefficients of thermal expansion of the components of the semiconductor device/circuit board interface, stresses in the underfill sealant occur (as contrasted to stresses in the semiconductor device and/or circuit board, had an appropriate underfill sealant not been used) during thermal cycling.
That is, thermosetting epoxy formulations when cured, are typically rigid and relatively brittle polymers with high modulus values. As such, much of the stress caused during thermal cycling is transferred to the CSP, BGA, LGA or FC assembly instead of the circuit board, resulting in cracking when the stresses become severe. While there are many commercially available flexibilizing agents, such as rubbers, thermoplastics, and diluents, that one may include to the formulation to improve flexibility by providing low modulus values, moisture absorption by the cured reaction product ordinarily increases as a result, often to an unacceptable degree.
Attempts at improving adhesion of such underfill sealants have often involved the inclusion of materials that would tend to decrease the crosslink density of the cured sealant. While such a decrease improves flexibility and thus oftentimes adhesion, it also has resulted in the increase of moisture absorption. Moisture absorption of such sealants is seen as a detriment to the overall function of the microelectronic device due to the increased chance of corrosion, and therefore the malfunctioning of the device.
UOP Corporation offers commercially under the tradename UNILINK a series of aromatic secondary diamines, which are promoted as useful in modifying the urea linkage in polyurethane and polyurea compositions. It is reported that the modification permits a greater amount of the diamine to be incorporated into the formulation, thereby resulting in a polyurethane or polyurea having superior strength and load bearing performance, as well as improved dimensional stability, as compared to foams prepared without the diamine. These diamines are not believed to have been promoted to date for use in epoxy-based formulations, such as ones not based on anhydride curing, let alone for the purpose of improving adhesion after exposure to elevated temperature conditions and resistance to moisture absorption.
U.S. Pat. No. 5,503,936 (Blyakhman) describes and claims curable modified epoxy resin compositions having an epoxy resin, a hardener or curing agent and 2.5 to 12.5% by weight of a compound represented by where E and T are C5-12 alkyl, C5-8 cycloalkyl, C7-15 phenylalkyl, or C6-10 aryl, with or without substitution by one or two C1-4 groups. The hardeners or curing agents of the '936 patent are described as aliphatic, aromatic or cycloaliphatic di- or polyamines, such as diethylenetriamine, N-aminoethylpiperazine, 4,4′-diaminodicyclohexylmethane, 4,4′-diaminodiphenyl sulfone, diethyldiaminotoluene, dicyandiamide, or guanidine; polycarboxylic acid anhydrides, such as phthalic anhydride or trimellitic anhydride; catalytic curing agents such as tertiary amines, imidazoles or complexes of boron trifluoride; difunctional and multifunctional phenols; or phenol or cresol novolac resins.
In addition, in “High Performance No-Flow Underfills for Low-Test Flip Chip Applications: Material Characterization”, IEEE Transactions on Components, Pack'g and Man'g Tech.—Part A, 21, 3, 450-58 (September 1998), C. P. Wong et al. describes the use of cobalt acetylacetonate as a curing catalyst for underfill materials based on certain epoxy resins and anhydrides. These curing catalysts are described as latent and are noted as having no noticeable concentration dependent effect on the final properties of the cured samples. See also International Patent Publication No. WO 98/37134. In the context of epoxy/cyanate ester curable compositions, see also U.S. Pat. No. 5,969,036 (Dershem).
In addition, U.S. Pat. No. 5,541,000 (Hardy) speaks to compositions having an epoxy resin, an aromatic polyamine curing agent and cure accelerator compounds, such as π-electron acceptors selected from neutral organic compounds, neutral coordination compounds, such as copper(II) trifluoroacetylacetonate, manganese (II) acetylacetonate, manganese(III) acetylacetonate, cobalt(IT) acetylacetonate, cobalt(III) acetylacetonate, and mixtures thereof, ionic coordination compounds, ionic organometallic compounds, quaternized iminothioethers, and aromatic cations, which have a formal charge on an endocyclic heteratom. These compositions are described as being useful in structural composites, filament wound articles, pultruded articles, film adhesives, and printed wiring boards. And the claims require at least a 1:1 stoichiometry between the epoxy and the aromatic polyamine curing agent, where the relative amount of the aromatic polyamine curing agent can increase beyond that stoichiometry, such as up to 1:1.8.
Finally, polysulfide-based toughening agents are known. However, their use to date is not believed to have been in thermosetting resin compositions, particularly those destined for microelectronic applications, where it has been desirable to demonstrate improved adhesion after exposure to elevated temperature conditions, improved resistance to moisture absorption and improved resistance to stress cracking. Rather, polysulfide-based toughening agents have been used to provide flexibility to materials used in general industrial applications and civil engineering/construction applications.
It would be desirable for an underfill sealant composition to provide good adhesive properties, such as flexibility, while at least maintaining the current level of, if not improving the, resistance against moisture absorption, while improving the stress cracking resistance of the cured product. With such physical properties of the cured product, CSP, BGA, LGA and/or FC assemblies should have improved reliabilities, all else being equal of course.